Rechargeable storage batteries are used in a wide variety of applications. Lead-acid batteries are one such type of rechargeable storage battery and are commonly used in automobiles, other vehicles, and in numerous back-up power systems. As such, these batteries experience a broad range of operating and storage temperatures. In northern U.S. climates, ambient temperatures often fall well below -10.degree. C. and in southern U.S. climates, ambient temperatures may exceed 50.degree. C.
Two other factors influence automobile battery temperature. Because automotive batteries are typically located in the automobile engine compartment, they are exposed to engine heat. When engine cooling is at a minimum, such as during city stop and go driving or while the engine is idling, heat generated by the engine may elevate the underhood air temperature above 100.degree. C. Moreover, the heat generated within the battery during use further elevates the battery temperature. Recent trends in vehicle design have resulted in even greater underhood temperatures. This is due to hotter engine designs built to improve engine efficiency and due to automobile downsizing, improvements in automobile aerodynamics, and an increased number of underhood components which restrict underhood air flow.
These battery, temperature extremes are of concern to automobile and battery manufacturers because the storage capability and usable life of these batteries is largely dependent upon battery temperature. At colder temperatures, the ability of lead-acid batteries to deliver current is decreased. Furthermore, the average running state-of-charge is typically lower at colder temperatures, due to decreased charge acceptance rates and the increased loads placed on the automotive charging systems. As a result, lead-acid batteries provide relatively poor cold starting performance, especially when the battery is used multiple times to restart the engine within a short period of time. At higher temperatures, the battery grids are subject to accelerated corrosion and battery water loss due to electrolysis caused by an excessive voltage and, in some instances, evaporation of the water from the acid. Such corrosion may create a protective layer on the battery grids, thereby reducing the amount of exposed plate material available for storing energy. Additionally, boiling and electrolysis of the battery acid loosens and separates plate material from the battery grids, thereby reducing battery storage capacity.
By controlling battery temperature within desirable limits, useable battery life can be more than doubled in some instances. It would therefore be useful to have a battery thermal control system that maintains battery temperature to within desirable limits. However, such a system preferably has several features. First, the system should be capable of both heating or cooling the battery, as necessary. Secondly, the system should operate independent of vehicle operation. This is especially true where the automotive battery is subject to engine heat, since the engine acts as a large thermal mass which can maintain the underhood temperature at an elevated level up to several hours after the engine is turned off. Thirdly, the system should exhibit very little power drain and should be sensitive to battery state of charge. The system should prevent discharging the battery beyond the point where the benefit of additional temperature adjustment is less than the deficit of a deeper discharge. Deep discharging of batteries is another contributor to decreased battery life.
Although many systems for changing battery temperature have been developed, none provide all the above-mentioned features required for proper battery thermal protection. For instance, U.S. Pat. Nos. 2,104,769, 2,104,773, 3,977,490, 4,976,327, and 5,015,545 disclose various methods for cooling an automobile battery. None of the apparatuses disclosed in these patents provide adequate thermal protection, because, among other things, they are not able to heat the battery to a temperature which provides a satisfactory battery charge acceptance. U.S. Pat. Nos. 2,717,045, 4,840,855, and 5,031,712 each disclose systems for selectively heating or cooling the automobile battery. Although some of these systems provide for heating or cooling of the battery when the engine is not running, none of these systems are, among other things, sensitive to battery state of charge to prevent undesirable discharging of the battery.